PROJECT SUMMARY/ABSTRACT The blacklegged tick Ixodes scapularis is the principal vector for infectious disease in the United States and transmits the Lyme disease bacterium Borrelia burgdorferi. Despite their public health importance, ticks are understudied and many of their immune mechanisms are poorly characterized. The recent discovery of a non- canonical immune deficiency (IMD) pathway in ticks highlighted the gap in knowledge of non-insect immune pathways. The Ixodes IMD pathway is one of the primary defenses against tick-borne bacteria and is stimulated by lipids derived from microbial infection. However, several of the molecular components of the pathway, including the pattern recognition receptor, remain elusive. In this F31 application, we report a candidate receptor for infection-derived lipids in ticks, which we demonstrate to be important for limiting B. burgdorferi infection. The central hypothesis of this proposal is that this receptor binds infection-derived lipids and relays antimicrobial signals in ticks. In Aim 1, the molecular interactions between the receptor and infection-derived lipids will be elucidated using sophisticated biophysical techniques. We will define these interactions by measuring binding affinity and stoichiometry, as well as determining critical residues for lipid binding. In Aim 2, we will examine this protein?s role in immunity through two approaches. As a predicted signaling component of the tick IMD pathway, we will observe its ability to regulate innate signal transduction. Furthermore, its role in pathogen engulfment and cellular trafficking will be investigated using state-of-the-art live cell imaging and confocal microscopy. By completing the aims of the proposed research, we will characterize a novel receptor in I. scapularis immunity and define molecular mechanisms of vector competence. Thus, these findings will contribute to new scientific paradigms in non-insect arthropod immunity and shed light on the evolution of vector-pathogen interactions.